China accelerated the depreciation of the yuan on Thursday, sending regional currencies and stock markets tumbling as investors feared the Asian giant could trigger competitive currency devaluations from trading partners.

China's stock markets were suspended for the rest of the day less than half an hour after opening as a new circuit-breaking mechanism was tripped for the second time this week.

The People's Bank of China (PBOC) again surprised markets by setting the official midpoint rate on the yuan, also known as the renminbi (RMB), at 6.5646 per dollar, the lowest since March 2011.

That was 0.5 percent weaker than the day before and the biggest daily drop since last August, when an abrupt near 2 percent devaluation of the currency also roiled markets.

Regional currencies promptly went into a tailspin. The Australian dollar, often used by foreign exchange dealers as a liquid proxy for the yuan, fell half a U.S. cent in a blink.

The PBOC's China Foreign Exchange Trade System (CFETS) repeated on Thursday that there was no basis for the yuan's continuous depreciation and that it was stable against a basket of currencies in 2015.

But the central bank's fixings have helped drive the yuan down not just against the dollar this week, but also other major currencies, including a 3.5 percent fall against the yen and 0.8 percent against the euro.

That raised concerns that China might be aiming for a competitive devaluation to help its struggling exporters.

"That's the fear of the market," said Sim Moh Siong, FX strategist for Bank of Singapore, adding that it was a zero sum game as other currencies weakened in response, and the end result would be greater volatility.

Others in the market were unsure what policy Beijing was pursuing.

"Frankly speaking, we are still not quite sure where the PBOC boundary is at the current stage. The fear of the unknown has become the largest risk for RMB in the near term, despite China’s sizable current account surplus," said Singapore-based Oversea-Chinese Banking Corporation (OCBC).

OCBC noted that against a basket of currencies, the RMB index was still only fractionally down for 2016, despite this week's fixes against the dollar.

ANZ bank said in a note that the PBOC's action would nevertheless "create one-way expectation of RMB depreciation, propelling capital flight and leading to significant financial instability".

This week's slide comes ahead of the expected release later in the day of China's foreign exchange reserve data for December, which traders fear will show a further sharp decline as investors pull money out of the slowing economy.

China's reserves, the world's largest, fell by $87.2 billion in November to $3.44 trillion, the lowest level since February 2013 and the third-largest monthly drop on record.

Some fear the yuan's quickening slide suggests the world's second-largest economy is in deepening trouble, though for now economists say there has been no material change in their expectations of a gradual but bumpy slowdown, with no hard landing.

Markets | Thu Jan 7, 2016 8:26am EST | BY TANYA AGRAWAL/ Reuters

Stock futures tumble as China allows yuan to fall further

U.S. stock index futures fell sharply on Thursday after China allowed the yuan to fall further and oil prices slid to near 12-year lows, raising concerns over the state of the global economy.

* China allowed the biggest fall in the yuan in five months, and Shanghai stocks slid 7.3 percent to trigger the second trading halt this week.

* With Beijing accelerating the yuan's depreciation to make its exports more competitive, investors fear China's economy is even weaker than had been imagined.

* Oil slid below $33 a barrel to near 12-year lows, as a tumble in Chinese equities rattled investors already concerned by near-record production and massive stockpiles of unwanted crude and refined products. [O/R]

* European stocks were also at their lowest levels since late August, when concerns over China's economy roiled global stock markets.

* U.S. stocks closed at their lowest level since early October on Wednesday.

* Shares of Apple were down 2.9 percent to $97.80, following reports of slowing shipments of the iPhone 6S and 6S Plus.

* Oil majors Exxon and Chevron were down more than 2 percent, while miner Freeport-McMoRan saw its shares fall 5.2 percent to $5.85.

* Tech majors Facebook, Amazon and Yahoo were all down more than 2.5 percent.

The halt mechanism, intended to calm market volatility, was having the opposite effect, according to a 22-year-old retail investor in Guangzhou surnamed Hu.

He said he bought shares on Wednesday when the market rebounded, but was now trapped by the circuit breaker, which he said was "killing investors" and creating panic.

China's securities regulator also unveiled new rules on Thursday to restrict selling by big shareholders who have been locked into their holdings for six months since Beijing banned them from offloading stocks to arrest a summer market crash.

In rules that take effect on Jan. 9, they can't sell more than 1 percent of a listed company's share capital every three months.

The new rules didn't go down well with investors.

“This is crazy. Chinese regulators set off on this path in July and they cannot get out of it. They have ruined whatever hope investors still had in the market," said Alberto Forchielli, founder of Mandarin Capital Partners.